I keep returning to the Infinity Engine games and NWN franchise for the coop experience. I think it adds a lot to the game experience.
The problem is that we are losing track of what players who play story driven RPG want out of MP. We want the ability to play with our friends and experience the story together. If that means designating one PC as the leader than so be it. Borderlands MP ruins the SP experience because the premise is to have everyone be a main character. Everyone loots, everyone talks, we all have money. MP can be a lot more restrictive in a story driven RPG and still be fun for everyone:
Lock PCs at game start. No adding or removing characters.
Make the party leader be the only one who can talk to NPC.
Make the party leader be the only one who can change zones.
Have 1 quest log for the party.
Have 1 common loot bag.
4 & 5 are features that I would like to have seen in BG, BG2, NWN, etc. Keep in mind that we are talking about LAN support. If you invite and @!#@& in your house, ... well kick him out.
I think it's too late for Project Eternity to have this type of Coop, but I hope that they allow for Mods to give basic MP support or that another kickstarter is initiated for this type of MP.